Inability of host immune system to develop and/or maintain antitumor response is one of the major factors responsible for tumor escape and low efficacy of cancer vaccines. In recent years it became increasingly clear the existence of two major paths of immune non-responsiveness to established tumors. First is a local path. In tumor milieu a multitude of different factors associated with tumor cells and tumor stroma create potent immune suppressive environment. This results in non-specific inhibition of T-cell function. The second path is a systemic defect in immune reactivity against tumor. Data accumulated in recent years demonstrated that the main component of this systemic defect is antigen- specific T-cell tolerance. Understanding the mechanisms of T-cell tolerance in cancer is critically important not only for better understanding of the biology of this disease but for the development of most effective therapeutic strategies. It has been previously shown that T-cell tolerance in cancer is caused not by tumor cells but by host antigen presenting cells. However, the exact nature of these cells and the mechanism of this effect remain unclear. Intensive studies during recent years have provided convincing evidence that myeloid-derived suppressor cells (MDSC) play a critical role in immune defects in cancer. These cells contribute to immune defect in local tumor microenvironment. However, their role in systemic defects is more important. We and others have demonstrated that MDSC induce antigen-specific T-cell suppression in cancer. Elimination of MDSC dramatically improved antitumor immune responses. Most recently in direct experiments in vivo MDSC were shown to cause CD8+ T- cell tolerance. This effect was specifically attributed to MDSC since neither immature myeloid cells from control mice nor myeloid progenies of MDSC could induce CD8+ T-cell tolerance. Our preliminary data described in this application have suggested a novel mechanism of T cell tolerance employed by MDSC. It proposes that MDSC via hyper-production of peroxynitrite induce nitrite modification of TCR and CD8 molecules on the surface of antigen-specific T cells. This process requires close cell-cell contact and the presence of specific antigen. Nitrate modification of TCR and CD8 alters T-cell recognition of the specific antigens. Thus T cells lose their ability to recognize specific antigens while retaining their response to non-specific stimuli. This mechanism may explain the phenomenon of tumor- specific T-cell tolerance. In three specific aims of this application we propose to investigate the mechanism and biological significance of these findings and evaluate their therapeutic potential. Specific Aim 1. Investigation the basic mechanisms of MDSC induced T-cell tolerance in cancer. Specific Aim 2. Investigation the mechanisms of altered antigen recognition by CD8+ T cells caused by MDSC. Specific Aim 3. Investigation the role of reactive oxygen species in immunosuppressive effects of MDSC and their therapeutic modification. PUBLIC HEALTH RELEVANCE: Proposed research will investigate novel mechanism of immune suppression in cancer. It will define the role of reactive oxygen in tolerance of T cells and the role of myeloid derived suppressor cells in this process. Proposed study has a goal of developing new therapeutic strategy aimed on improvement of immune responses in cancer and enhancement the effect of cancer vaccines.